Compression-ignition internal combustion engines typified by diesel engines are internal combustion engines in which air or another gas to be reacted with fuel is highly compressed, fuel is introduced into the high-temperature, high-pressure atmosphere, the fuel is dispersed to form a so-called mixture, and the mixture is made to self-ignite. Since the compression-ignition internal combustion engine can, in theory, use various types of fuel, the value of such engines as low-pollution engines is being reassessed. However, there is a problem in a diesel engine in that substances that cause air pollution are generated in large quantities when combustion is not carried out in optimal conditions. For example, the combustion reaction is incomplete when a load is suddenly applied, and particulate matter (PM) is produced. On the other hand, the combustion temperature is increased and nitrogen oxides (NOX) are produced when an attempt is made to achieve a complete combustion reaction. In a diesel engine, ignition is achieved by injecting fuel using an injector into high-temperature, high-pressure air, but in principle it is difficult to control the ignition delay time, the spread of the flame, the amount in which exhaust gas is generated, and the uniformity of the areas of higher temperatures.
Also, in a diesel engine, air must be compressed at a higher pressure than in a spark-ignition engine during the compression stroke because the thermal energy required to ignite the fuel is obtained by adiabatic compression of the air in the cylinder, and the energy required in the compression stroke is greater by a commensurate amount than spark ignition.
There are widely known techniques for solving such problems of a diesel engine, as well as various other problems related to combustion in a compression-ignition internal combustion engine, examples of which include forming members that enclose the combustion chamber in a shape that produces a squish flow, swirling, or tumbling in the working fluid in the combustion chamber to facilitate the formation of a mixture by using the squish flow, swirling, or tumbling.
For example, there is also a technique referred to as common rail. This is a scheme in which fuel is pressurized inside a pipe referred to as a rail, and fuel injection is carried out by opening an electric valve provided to the injector. Since the fuel spray function is separately provided from the pump, it is possible to achieve fuel pressurization having a high valve-opening pressure in excess of 180 MPa, and to achieve spraying or another type of high-level spray control over a plurality of cycles. It is also possible to reduce NOX, PM, and the like by high-level spray control and by the effect of shortening the time for mixing the fuel and a combustion aid by high-speed fuel jet flow.
Combustion enhancement that uses plasma is also receiving attention as a technique for solving various problems related to combustion in a compression-ignition internal combustion engine.
A typical combustion enhancement technique that involves a method for using plasma is disclosed in Patent Document 1. The diesel engine described in Patent Document 1 is provided with a discharge electrode in the combustion chamber. The discharge electrode forms plasma by corona discharge when combustion by self-ignition occurs. Combustion is enhanced in the diesel engine by electrifying and ionizing the fuel using plasma.
The present inventors developed a technique for enhancing combustion of a mixture in a self-ignition internal combustion engine by using microwave emissions as described in Patent Document 2. In this technique, microwaves are emitted in the combustion/reaction region, and large quantities of OH radicals and ozone (O3) are generated from the moisture in the mixture in the combustion/reaction region. Means for inducing self-ignition causes the mixture to ignite, and combustion of the mixture in the combustion/reaction region is enhanced by large quantities of OH radicals and ozone.    [Patent Document 1] Japanese Laid-open Patent Application No. 2001-317360    [Patent Document 2] Japanese Laid-open Patent Application No. 2007-113570